Pumps for use with poisonous, inflammable, expensive and/or environmentally unfriendly liquids as pumping medium can be constructed to be substantially completely leakproof due to the axle on which the pump fan or impeller is fixed being arranged in a medium tight shell.
In many known pumps it is important, for cooling and lubrication purposes, that the axle bearings lie in the flow of the medium to be pumped. For example, when a pump has been stationary for some time, gas and air bubbles can congregate at certain places in the pump, which leads to insufficient cooling and/or lubrication, as well as increased wear, of the bearings.
In British patent specification GB-A-869781 a fuel pump is disclosed which is to be mounted in a normally submerged position within a fuel tank, e.g. in the wing of an aircraft. The stator of an electric motor, which drives a rotor which is operatively connected to an axle of the impeller, is enclosed in a tube or shell for sealing this stator from the fuel. The rotor is also enclosed in a medium tight package or shell, so that the safety of operation of the pump is secured. The pumphouse or casing is provided with a vent at the upper end thereof for discharging air, vapor and fuel into the interior of the tank in which the pump is disposed. Maintenance of the electric motor is difficult or even impossible. Any bearing for the axle will deteriorate in a relatively short period of time because of increased wear due to lack of lubrication and cooling thereof.
In German Gebrauchsmuster DE-U-9116052 a pump is disclosed, in which a rotor part operatively connected to the axle for the impeller is provided with first magnetic elements which are coupled to second magnetic elements. The second magnetic elements are driven by a driving motor and provide for movement of the first magnetic elements and the axle. A fluid tight shell is provided between the first and second magnetic elements. To achieve a sufficient cooling and lubrication of the bearings of the axle, an additional booster pump member is disposed onto the same axle as for the main impeller.